Cathode structure



Nov. 28, 1950 L. A. WILLIAMS CTHODE STRUCTURE Filed May' 26, 1948 IIIIIII Patented Nov. 28, 19.50

CATHODE STRUCTURE Louis A. Williams, Belmont, Mass., assigner to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application May 26, 1948, Serial No. 29,312

14 Claims.

This invention relates to cathode structures, and more particularly to an indirectly-heated cathode for electron discharge devices of the socalled magnetron type.

It has been found advantageous to use, in modern high-power tubes of the type referred to, cathodes which utilize the electron-emitting properties of thoria (thorium oxide), since emitters of this material can readily provide the copious electron emission so necessary for tubes of high power. However, electron emitters of thoria must be run or operated at rather high temperatures, on the order of 1500 C. for example, in order for them to function effectively and efciently.

In prior art structures, in order to obtain temperatures of this order at the emitting surface, it was necessary to run or operate the heater itsalf at a much higher temperature, one on the order of 2500 C. or even higher. The necessity for this was due in part to the high rate of conduction of heat, away from the emitting surface, by adjoining or adjacent metallic surfaces, and due in part, if insulated heater wires were used, to the relatively poor heat-conducting characteristics of the insulating coating.

These very high temperatures at which it was necessary to run the heater, in prior art structures, shortened the life of such heaters to a substantial extent, and produced other deleterious and undesirable effects on such heaters. It has been found that insulated heater wires are impractical for such applications, since the insulating coating will not effectively withstand such temperatures for a sufficiently long period of time. If bare heater wires are used, they will start to sag in a very short time when operated at such high temperatures, and this is very undesirable, since they then come into contact with their cathode sleeves and cause short-circuits.

In addition, if the heater of a magnetron is supplied with alternating current, vibrations of such heater can be produced, as a result of the interaction of the heating current with the magnetic eld necessarily existing in the magnetron. Such vibrations are undesirable for several reasons, and it is therefore necessary to so support the heater as to substantially prevent vibrations thereof.

Accordingly, the principal object of this invention is to devise an indirectly-heated cathode structure for magnetrons which has a long usable life, on the order of 500 hours for example, even when operated at a high temperature in tubes of high power, and a structure which effectively overcomes the above-described difiiculties. This object is achieved, according to the present invention, by devising a cathode structure in which there is only a relatively low temperature differential, between the heater and the emitting surface of the cathode, when the device is in operation. For example, if the emitting portion of the cathode is to be operated at approximately 1500 C., according to this invention the heater need be operated at a temperature of only 2000 C. or below.

Another object of this invention is to devise a novel thoria cathode structure for high-power magnetrons.

The foregoing and other objects of the invention will be best understood from the following description of some exemplifications thereof, reference being had to the accompanying drawing, wherein:

Fig. l is a vertical section through a cathodepole piece assembly according to the invention;

Fig. 2 is a section taken on line 2 2 of Fig. 1, on an enlarged scale;

Fig. 3 is a section taken on line 3-3 of Fig. 1;

Fig. 4 is a vertical section through a cathode assembly of modified construction;

Fig. 5 is a horizontal section taken on line 5-5 of Fig. 4; and

Fig. 6 is a perspective view of the upper por tion of one of the members of Fig. 4, as it appears during the process of manufacture.

Now referring to Figs. 1-3, which illustrate one embodiment of a cathode structure according to this invention, a cylindrical metallic sleeve E, of molybdenum or other suitable refractory metal, serves as a rigid supporting member for the emissive portion of the cathode, said sleeve preferably being machined from seamless tubular stock. A reticulated or screen-like mesh member l surrounds sleeve 5 and is rigidly secured to the same, preferably by being brazed or sintered thereto with molybdenum powder at a high temperature, on the order of 2300 C., for example. I have found that, when members 6 and l are joined in the above-described man-- ner, screen member 'I is secured to sleeve 6 in an intimate manner. Sleeve 6 has a length in excess of that of screen member 1, and said screen member surrounds the central portion of said sleeve. Screen 'l supports and holds thereon a suitable electrcn-emissive coating 8 of thoria (thorium oxide) which is coextensive with said screen. In the above-described manner, emissive coating 8 is rmly held on sleeve 6.

A plurality of equally-spaced, circumferentially-extending horizontal milled slots 9 (see liig.`

3), here shown as four in number, are provided in sleeve 6 slightly below the lower end of screen 1, the center lines of these slots all lying in a common horizontal plane. A similar plurality of equally-spaced, circumferentially-extending horizontal milled slots it, four ink number,` are provided in sleeve E slightly above the upper end of screen I', the center lines of these slots also all lying in a common horizontal plane. As illustrated in Fig. 3, there is only a small cross-section of metal between adjacent slots 9 and Iii, so that the far larger part of the cross-sectional area of sleeve t in the planes of these slots consists oi poorly-heat-conducting space. The operating temperature of the active or emitting portion 'I-B of cathode sleeve 6 is intended to be rather high, on the order of 1500" C., temperatures of this order being necessary in order to produce copious emission of electrons from. the active materiali 8i utilized. It is desirable, for manyv reasons, to prevent heating of. the end portions of sleeve E, or of adjacent portions of the cathode structure, to thishi'gh temperature. The transverse slots Il and IEB, at the opposite ends of the central' portion of sleeve S, provide a heat choke, heat shield, or heat interrupter to mini'- mize the conduction of heat, from the heated central portion oi sleeve 6, to the end portions of said sleeve. Since the cross-section of' heatconducting metal: between the end and central portions of supporting sleeve t is held to an absolute minimum by slots 9: and lli, the conduction of heat from the central to the end portions of said sleeve isflimited to a very small amount, so that said end portions remain cool relative toisaid central portion, when said central portion is heated for electron emission.

A bottom support member II, in the form' of a hollow open-ended cylinder having an integral transversely-extending partition Ha therein; is secured as by brazing to the lower end portion of sleeve 6, the lower end face of said'sleeve contacting the upper surface of partition IIa. The upper part oiv member I-i is reamed out to a larger diameter than'the outer diameter of sleeve 6, to thereby provide an insulating annular space between the outer surface of said sleeve and the innersurface of said member in the region of slotsv 9, the outer surface oi said sleeve iirmly engaging the inner surface o said member in the region below saidv slots. The'len'gthof member Ii is such that the upper end thereof is 1ocated just' slightly below the lower end or screen 'I and coating S, and at its upper endY member II, which is machined from seamless tubular metallic stock, is provided with an integral outwardly-eXtending horizontal circular flange IIb, to Serve as an end shield, preventing emission of electrons from coating Y in undesired` downward directions. Due to the facts that member II is metallically attached to sleeve Ei only in an area below and substantially distant from heat-choke slots 9, andthat there is an annular space between the outer surface of said sleeve and the inner surface' ofi` said member substantially throughout thelength of said member, the conduction' of heat away from the emitting portion 1 8 of the cathode, by member il', is very substantiallyireducedas compared to prior art structures:v In fact, such conduction is` reduced to substantially zero, or at" least' to a very low rate.

Anupperf support member It', in the form ofa hollow open-endedv cylinder having an internalV annular projection IZ'a. ltherein, is secured as by brazing to the upper end portion of sleeve Ei, the upper end face of said sleeve contacting the lower surface of projection I2a. The lower pariI of member I2 is reamed out to a larger diameter than theouter diameter of sleeve E, to thereby provide an insulating annular space between th outer surface of said sleeve and the inner surface of said member in the region of slots Il, the outer surface of said sleeve firmly engaging the inner surface of said member in the region above said slots. The length of member I?? is such that the lower end thereof is located just slightly above the upper end of screen 'l and coating l, and at its lower end member I2, which is machined from seamless tubular material stock, is provided with an integral outwardiy-extending horizontal circular ange i219, to serve as an end shield, preventing emission of electrons from coating 8 in undesired upward directions. Membernv i2 isthus` metallically attached to sleeve 6 only in anv area above and substantially distant from heat-choke slots Iii, and there is anl annular space between the outer surface of said sleeve and: the-inner surface of said member substantially'throughout the length of said member. Therefore; the conduction of heat away from the emittingy portion' of. the cathode, by member I2, is very substantialy reduced as compared toprior artstructures, and is in fact reduced to substantially zero.

An elongated supporting cylindrical metallic sleeve I3 isfrmly secured at itslower end, as by brazing,Y in andV to: the upper endV of member I2, the lower end face of sleeve I3 contacting the upper face of projection I2a and the outer surface of sleeve I3 contacting the inner surfaceoi' member: I2; Somewhat above the joint between sleeve I3 and member I2, said sleeve is provided with: a plurality; for example three, of equallyspaced', circumierentially-extending horizontal milledV slots lli', the centerlines of these slots all lying in a common horizontal plane. Sleeve I3, similarly to members 6, Il' and I2', is machined from seamless tubular stock, and slots Ill serve as an additional heat choke or heat interrupter to further. inhibit the conduction of heat from the hot central portion of sleeve 5 to the portions of the'cathode structure above slots` it, thus preventing` said last-named portions, which are sealedw orV fused to glass? members, from reaching anV undesirably high temperature.

The outer wall of sleeve I3 is provided with two horizontal annular shoulders near the upper end of said sleeve, and Asecured a'sby brazing atk and to the uppermost of these shoulders is a sealingV metallic sleeve I5 which is made of an alloy, suchas Kovar, which is ametal commonly used forr sealing to glass, the lower end surface or sleeve I5 contacting the horizontal face of the uppermost shoulder on sleeve I3. A'- hollow glass insulating sleeve I6 is iused at its lower end to the'upper end= of sleeve I5, while the upper end of sleeve I6 is in turn fused to the lower end of a second sealing metallic sleeve I1, also preferably made of Kovar.

The enlarged upper end of a rigid metallic solid4 rod heater conductor I8y passes through sealing sleeve I'I, the upper end of said sleeve being hermetically sealed to the said end of conductor I8 around the periphery thereof, in order to hermetically seal and support the upper' end ofconductor i8 centrally of hollow members 6', I25, I3, I5, I6 and I'I. A- ilexible lead wire 42 is ellall'y connected to the upper (exterior)` end of conductor I8. By means of sleeve I6, conductor I8 is eletcrically insulated from members 6, I2, I3 and I5. Rod I8 is preferably machined from solid stock, and terminates at its lower end, somewhat above slots I0, in an integral externally-threaded stud portion I9, which is more clearly shown in the embodiment of Fig. 4.

A bare or uncoated heater wire 20, of tungsten or other refractory metal, is coiled in the form of a helix and is secured and electrically connected to the lower end of rod I8 by having a plurality of turns at the upper end of said coil wound into or threaded into the thread grooves of stud portion I 9, as clearly shown in Fig. 4. I have found that, by this threaded or wound coupling or joint between the heater wire 20 and rod I8, particularly if the heater wire is wound or threaded on an external thread, an extremely strong rigid joint or coupling may be made between said wire and said rod, without the necessity of any other fastening process, such as welding or brazing, being performed. Thus, a rather simple yet very strong joint between the upper end of the heater wire and rod IB is obtained. The pitch of the turns of heater coil 20 is greater than the pitch of the threads or stud portion I9, so that the turns of said coil are compressed when wound on stud I9 and thus tend to spread apart after the joint between heater 20 and rod I8 is made, thus tending to -cause the turns of coil 20 to bind in the threads more securely. Due to the rigidity of the joint between the upper end of heater 20 and rod I8, and also to the rigidity of the joint between rod l 8 and sleeve I'l, the upper end of the bare heater wire 2B is firmly held centrally of sleeve 6 and is eifectively prevented from coming into contact therewith.

In order to secure the lower end of heater coil 20 in position, and to provide an electrical connection to the same, a plurality of turns at the lower end of said coil, somewhat below slots 9, are wound into or threaded into the thread grooves of an externally-threaded headless metaln lic member or stud 2|, as clearly shown in Fig. 4, the pitch of the turns of said coil again being greater than the pitch of the threads on stud 2I. An extremely rigid and strong joint, similar to that between the upper end of heater 20 and rod I8, is thus provided .between the lower end f of said heater and stud 2I. Stud 2I is threaded throughout its length, and below the lower end of the heater it is threaded into a tapped aperture which is located centrally of partition I Ia. When stud 2I is tightened in said aperture, the lower end of heater coil 20 is supported rigidly with respect -to member II. The heater coil 20 is made of sufficiently large diameter wire to be rather rigid in itself, and is supported rigidly at its upper and lower ends by threaded joints, without the use of any additional fastening means, in such a way that this bare wire heater coil is rigidly mounted centrally of sleeve 6, closely adjacent thereto yet spaced therefrom, without any possibility of its contacting said sleeve. Throughout the central portion of the length of sleeve 6, or the emitting portion 1 8 of the cathode, heater wire 2) is in effect hollow, or, in other words, there are no members of any kind to inhibit the free flow of heat from said heater wire to sleeve 6 which supports coating 8.

Since heater Wire 20 is in effect hollow throughout the emitting portion 1 8 of the cathode, and since the end shields or members II and I2 are mounted in substantially non-heat-conducting sembly suitable for use in relationship with respect to portion 1 8 of the cathode, as above described, there is extremely good conduction of heat from the heater to the cathode, and extremely poor conduction of heat away from the exterior of the cathode. Therefore, during operation of the cathode there is only a low temperature difference between the heater 20 and the cathode 6. For example, to operate the cathode at 1500o C., the heater need be operated at a temperature of only 2000o C. or below. The fact that the heater wire in this construction is bare, and not covered with a heat-insulating or poorly-heat-conducting coating, also increases the conduction of heat from the heater to the cathode.

Due to the rigid mounting of the heater 20 as above described, bare or uncoated heater wire may be used without danger of short-circuits. Therefore, the cathode may be run at high op- I erating temperatures, above the temperature of breakdown for insulating coatings used in the prior art on heater wires, since such breakdown temperature was a limiting factor for previous structures.

Furthermore, due to the rigid mountings at opposite ends of the heater wire, combined with the longitudinal rigidity of the supporting structure, substantially no movement of the heater can occur in the longitudinal direction.

In order to provide a common heater-cathode electrical connection, a metallic ring member 22 is hermetically sealed around the periphery of sleeve I3, at the lowermost of the two horizontal annular shoulders thereon. A flexible lead wire 23 is electrically connected, as by soldering, to member 22. When a source of heater voltage is applied between leads 42 and 23, current ilows through rod I8, heater 28, stud 2I support member I I, sleeve 6, support member I2, sleeve I3, and ring member 22,l such current being converted into heat in heater 20, raising the temperature of sleeve 6 to the temperature of thermionic emission for coating 8.

In order to provide a cathode-pole piece asa magnetron, an inverted cup-shaped member 251, of the material known Kovar for example, is hermetically sealed horizontally around the periphery of ring member 22, and extends downwardly therefrom. The lower end of member 2d is fused and hermetically sealed to the upper end of a glass insulating sleeve the lower end of which is fused and hermetically sealed to the upper end of a Kovar sleeve 2E. rEhe lower end of sleeve Z'e in turn hermetically sealed into the central portion of an inverted frustro-conical pole piece 2l.

In order to mount the assembly as above ole-- scribed in s, magnetron to serve as the cathode thereof, pole piece 2l is hermetically sealed into a magnetron envelope, as more fully described in the copending Brown application, Ser. No. 742,541, filed April 19, 194:7. The glass sleeve 25 insulates the cathode sleeve I3 and lead 23 from the envelope or anode of the magnetron and pole piece 2.

Figs. l-S disclose a cathode structure the principal elements 93, Il, I2 and I3 of which are machined from seamless tubular metallic stock. lt is within the scope of this invention to nrostructure in which corresponding elements are stamped out of sheet stock and are thereafter rolled to cylindrical shape, thus providing a substantially cheaper structure than that previously disclosed. Figs 4-6 illustrate such a sheet metal construction, and in these gures, parts the same astthose* previously described are` denoted: by the saine; referencev numerals;

An. elongatedl metallicv sleeve 225i is stampe-:l out. of: reiractoryA sheet', metal and thereafter is rolled"into;openendedv cylindrical shape', as ndiin 6', until itsedges abutto forni a longitudinal seam.V 253. A'. horizontal' annular shoulder i isprovidedatthe upper end of sleeve 23; thisshoulder corresponding to thev one which ring. member: 22 contacts in Fig. li A screen member: 6. surrounds and'islf'astened. to sleeve 23 in the-same manner as in the embodiment pren viously describedrthis screen member properly supporting an erriissivecoatingV 'or'the type previously described.

A plurality, for.` example three, of equallyspace-d'; ci-rcumf'erentially-extending horizontal heat-choke slotsv 3.1? are punched' throughv sleeve 2-8-slightlyfbelow the lower end of screen l, the centerlines.oft'heseslotsk all lying in a common horizontal2 plane. asimilar plurality, for exampler three, of equallyespaced, circumferentiallyextending horizontal' heat-choke slots 32' are punched through sleeve 28 slightly above the Vupper. end' orf screen the center lines or these slots ail'l ina common horizontal plane. A third? group of' similar horizontal heat-choke slots 331is punched through sleeve 28 a .substantial distanceV above; the slots 32, while a fourth group orsiinil'ar' horizontal heat-choke slots Sil is punchedthrough sleeve 2g somewhat abovesiots 33, thesl'ots of' each group lying in av co unen horizontal plane and the slots or" group 35 il erably being displaced angularly with respect to those off group 332 As shown in Fig. 5, there is onlyl a; very small cross-section oi' heat-conductingl metal@ between adjacent slots in each group.

A' lower sheet metal cylindrical spacil sleeve 35- surrounds andVv braced to sleeve d at the lowerend thereof. Abottom sheet metal end cap 3d", rolled intheform of a cylindrical sleeve, surrounds and is braced: to the outer surface oi sleeve 35, sleeve 3S having a length such that it terminates just below the lower end of screen l. In order to provide a bottom end shield for the cathode, a piece of round wire 3l surrounds and is brazed to cap 3S at the upper end thereof, to prevent emission of electrons in undesired downward directions.

A similar upper sheet metal end cap structure is provided, including an upper sheet metal cylindrical spacing sleeve 3 which surrounds an is braced to sleeve i8 above slots 32. An upper sheet metal end cap 3a, rolled in the form o a cylindrical sleeve, surrounds and is brazed to the outer surface oi sleeve 33, sleeve 3S having a length such that it terminates just above the upper end of screen 1. A piece of round wire it? surrounds and is bra-Zed to cap 39 at the lower end thereof, to prevent emission of electrons in undesired upward directions. Spacing members 3% and 38 provide the necessary and desirable insulating annular space between sleeve 23 and the respective capV members 3S and 39, in the region of the respective slotsl and 3?.. End caps 3o. and 35, like the similar elements li and E2 in Fig. l are metallically attached to sleeve 28 only in areas substantially distant from the respective heatchokeslots 3| and' 32, on the sides of said slots respectively'opposite to the central portion of the cathode, and there are corresponding annular spaces between` the outer-surface'of the sleeve 2S and thev inner surfaces of caps 3% and 39, substantially throughout the length of such caps.

Therefore;. the conduction of heat awayffrom the emitting portionz-3 of the cathodaby caps 3% and 35h-.is reduced to substantially zero.

Theupper end of heater coil 2@ isioined-to rod iby meansof threaded studY portion i9", in the manner previously described, while rod i8 is mountedA centrallyin sleeve 2S in the same manner as previously described.

A'Y horizontal' bushing membern ill, having a central tappedaperture therein, is brazed inside the sleeve 28 nearV the lowerend thereof; The lower end of coil is threadedly secured to a stud-2 i, in the manner previously described, said studY being threadedV externally throughout its length and threaded-` tightly. into the central tapped aperture in bushing dfi.

As described` above in connection with the previous` embodiment; inthis wayV the heater coil 2!) is supported rigidly at its upper and lowerl ends by'these threaded joints, in such a way that this bare wire heater coil is rigidly mounted centrally of sleeve 28, closely adjacenty thereto yet spaced therefrom, without'any possibility of its contacting saidI sleeve.: Moreover, these joints atY the upper and lower ends of the heater coil 2o are mad-e without the necessity of; doing-any'welding or bracing, or oprovidingV any additional fastening means.

As inthe embodiment ofEig. 1, due to the construction ofthe cathode according to-this invention, there is extremely good conduction ofA heat from the heater to the cathode, and extremely poor conduction of heat away from the exterior of the cathode. During operation of the cathode thereY is only a low temperature diierence between the heater andthe cathode.

Electrical connections to the heater andcathode of Figs. 4-6 are made in the same manner as in the previously-described embodiment. A cathode-pole piece assembly is preferably constructed from the rolled sheet metal embodiment of Figs. 4 6 in exactly the same way as described previously yfor the machined embodiment of Figs. 1-3.

Ofl course, itis tobe understood'.A that this invention is not limited to the particular details as describedalbove, asmany equivalents'will suggest themselvesV to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with thescope of this invention within the art;

What is claimed is:

1. An indirectlysheated cathode structure, comprising a cylindrical supporting sleeve of ren fractory material, an electron-emissive coating supported on the outside of said sleeve, a rigid metallic member insulatingly supported substantially centrally inside said sleeve and spaced from the inner Wall thereof, and a bare wireheater having one end thereof secured to said member and its opposite end threadably fastened with respect to one end of said sleeve, said heater being disposedV opposite said coating and being spaced throughout its length from the inner Wall of said sleeve.

2. An indirectly-heated cathode structure, comprising an elongated cylindrical supporting sleeve, an electron-emissive coating supported on the outside of said sleeve over a portion of its length intermediate the ends thereof, said sleeve being provided with means for inhibiting the flow of'heat from said portion to the two end portions thereof, a rigid metallic member insulatinglysupported substantially centrally insidel said sleevel and spaced from the inner wall thereof, and a bare wire heater having one end thereof secured to said member and its opposite end rigidly threadably fastened with respect to one end of said sleeve, said heater being disposed opposite said coating and being spaced throughout its length from the inner Wall of said sleeve. v

3. An indirectly-heated cathode structure, comprising a cylindrical supporting sleeve of refractory metal, an eleotron-emissive coating supported on the outside of said sleeve, a metallic member insulatingly supported substantially centrally inside said sleeve, a supporting conducting member extending transversely across the interior of said sleeve near one end thereof, a bare Wire heater having one end thereof threadably secured to said metallic member, and means rigidly securing the opposite end of said heater to said supporting member, said heater being disposed opposite said coating and being spaced throughout its length from the inner wall of said sleeve.

li. An indirectly-heated cathode structure, comprising a cylindrical supporting sleeve of refractory metal, an electron-emissive coating supported on the outside of said sleeve, a rigid metallic member insulatingly supported substantially centrally inside said sleeve and spaced from the inner wall thereof, said member having a threaded portion at one end thereof, and a bare wire helical heater coil having a plurality of turns at one end thereof wound into the thread grooves of said threaded portion and having its opposite end rigidly metallically fastened with respect to one end of said sleeve, said coil Ibeing disposed opposite said coating and being spaced throughout its length from the inner Wall of said sleeve.

5. An indirectly-heated cathode structure, comprising a cylindrical supporting sleeve of refractory metal, an electron-emissive coating supported on the outside of said sleeve, a rigid metallic member insulatingly supported substantially centrally inside said sleeve and spaced from the inner Wall thereof, a bar wire helical heater coil having one end thereof rigidly secured to said member, and a threaded member at the opposite end of said heater coil into the thread grooves of which a plurality of turns at said opposite end of said coil are Wound, said threaded member being rigidly metallically fastened with respect to one end of said sleeve, said coil being disposed opposite said coating and being spaced throughout its length from the inner wall of said sleeve.

6. An indirectly-heated cathode structure, comprising a cylindrical supporting sleeve of refractory metal, an electron-emissive coating supported on the outside of said sleeve, a rigid metallic member insulatingly supported substantially centrally inside said sleeve and spaced from the inner Wall thereof, said member having a threaded portion at one end thereof, a bare Wire helical heater coil having a plurality of turns at one end thereof Wound into the thread grooves of said threaded portion, and a threaded member at the opposite end of said heater coil into the thread grooves of Which a plurality of turns at said opposite end of said coil are Wound, said threaded member being rigidly metallically fastened With respect to one end of said sleeve, said coil being disposed opposite-said coating and being spaced throughout its length from the inner wall of said sleeve,

7. An indirectly-heated cathode structure, comprising a cylindrical supporting sleeve of refractory metal, an electron-emissve coating supported on the outside of said sleeve, a rigid metallic member insulatinguly supported substantially centrally inside said sleeve and spaced from the inner Wall thereof, a supporting conducting member extending transversely across the interior of said sleeve near one end thereof, a bare Wire helical heater coil having one end thereof rigidly secured to said first-named member, and a threaded member at the opposite end of said heater coil into the thread grooves of Which a plurality of turns at said opposite end of said coil are Wound, said threaded member being threaded into said supporting member, said coil being disposed opposite said coating and being spaced throughout its length from the inner Wall of said sleeve.

8. An indirectly-heated cathode structure, comprising a cylindrical supporting sleeve of refractory metal, an elect1on-emissiVe coating supported on the outside of said sleeve, a rigid metallic member insuiatingly supported substantially centrally inside said sleeve and spaced from the inner wall thereof, said member having a threaded portion at one end thereof, a supporting conducting member extending transversely across the interior of said sleeve near one end thereof, a bare wire helical heater coil having a plurality of turns at one end thereof wound into the thread grooves of said threaded portion, and a threaded member at the opposite end of said heater coil into the thread grooves of which a plurality of turns at said opposite end of said coil are Wound, said threaded member being threaded into said supporting member, said coil being disposed opposite said coating and being spaced throughout its length from the inner Wall of said sleeve.

9. An indirectly-heated cathode structure, comprising an elongated cylindrical supporting sleeve of refractory metal, an electron-emissive coating supported on the outside of said sleeve over a portion of its length intermediate the ends thereof, said sleeve being provided with means for inhibiting the iovv of heat from said portion to the two end portions thereof, a rigid metallic member insulatingly supported substantially centrally inside said sleeve and spaced from the inner wall thereof, said member having a threaded portion at one end thereof, a bare wire helical heater coil having a plurality of turns at one end thereof wound into the thread grooves of said threaded portion, and a threaded member at the opposite end of said heater coil into the thread grooves of which a plurality of turns at said opposite end of said coil are wound, said threaded member being rigidly metallically fastened with respect to one end of said sleeve, said coil being disposed opposite said coating and being spaced throughout its length from the inner Wall of said sleeve.

1G. An indirectly-heated cathode structure, comprising an elongated cylindrical supporting sleeve of refractory metal, an electron-emissive coating supported on the outside of sleeve over a portion of its length intermediate the ends thereof, said sleeve being provided with means for inhibiting the flovv of heat from said portion to the wo end portions thereof, a pair of opposite metallic shield members secured to the outer surface of said sleeve adjacent the respective opposite end portions thereof and on the sides of the respective inhibiting means rei-note from said first-named portion of said sleeve, said members each extending toward said first-nani portion of said sleeve and being spaced from the answers ifi outer surf ace -thereof and hea-ter Ameans -mounted inside said sleeve.

4l1. An indirectiyeheated cathode structure, comprising an elongated vcylindrical supporting sleeve of `refractory metal, an electron-emissive coating supported on the outside of said sleeve overa portion of its length intermediate `the ends thereof, said sleeve having therein at opposite end s 0j said portion slotsfor inhibiting Ithe flow of yheatiroinsaid portion -to the two end portions thereof, a Ypai-r of opposite metallic shield mein- `p ers' securedto the outer surface of said-sleeve adjacent 4the Vrespective opposite end portions thereof and on the sides of -the respective slots reinot-e from said first-named portion of -said sleeve, said members each extending toward said first-,named --portion of said sleeve 4and 'being spacedirornthe outer surface thereof, and heater means mounted inside said sleeve.

`12. An indirectly-heated cathode structure, comprising an elongated cylindrical supporting sleeve of refractory metal, an electron-emissive coating supported on the outside of said sleeve over -apor-tion of its lengthintermediate the ends thereof, said sleeve-being provided with means for inhibiting the iow Yof heat `from V`said portion to the two end portions thereof, -a pair of yopposite metallic shield members secured -to the outer surface Vof said sleeve adjacent the respective opposite-end portions thereof and on the sides of the respective vinhibiting means remote (from said first-named portion of said sleeve-saidememhers each extending toward said `first-named portion 4oi saidsleeve and being'spaced from the outer surface thereof, a rigid metallic member insulatingly supported substantially-centrally iniii) sigle said sleeve and spaced from the inner wall thereof, and a-baize 'wire heater having one lend thereof Irigidly secured to said member and its opposite end rigidly threadably fastened with respect :to-one-end `ofsaidsleevefsaid heater being disposed opposite said coa-tingV and vbeing spaced throughout its length from the inner wall-of said sleeve.

`173. A cathode structure comprising -a cathode cylinder, a heater coil positioned within said cathode structure, and l-a -coil support comprising af-threaded conduotorfaplurality of Tturns-of said coil 1Abeing wound 'into lthe thread grooves of -said conductor.

14. -A Acathode heater structure comprising a heater coil, and a coil lsupport rcomprising -a threadedfconductor, -a lplurality of Lturns of said coil'beingwoundiinto =the:thread grooves of said conductor.

LOUIS A. WILLIAMS.

:REEERENGES v,C ITEI) ffhe following references are of record in the file of this Apatfmtf UNI'EED STATES .PATENTS Number Name Date `1,267,827 Whitney May 28, 191,8 :1,603,209 Payne, J-r -fOc-t. .12, 1926 1,9875 11 IRobinson Jan. 15, -1935 1,995,702 Capicotto Man 26, .1935 -`2,258,836 Willner Oct. :14, 194i 2,416,899 Brown fMar. 4, 1947 2,466,922 Wax Apr. 12, 11949 V2,458,129 `Spencer Apr. 26, :1949 2,501,089 l'Pomeranian Mar. 21, '1950 

